A year after the Madrid train blasts, Spanish investigators believe they have clarified key aspects of the country's worst terrorist attack.

Last month presiding Judge Juan del Olmo made public forensic evidence from a criminal probe which has generated nearly 40,000 pages of legal documents.

The attacks, which left 191 dead, are believed to have been carried out by a radical Islamist network based in Spain - mainly of North African origin.

According to Judge del Olmo's initial

conclusions, three groups were involved in the preparation and execution of the bombings.

The judge also believes that the attacks were a response to the previous conservative government's support for the war in Iraq.

Prosecutor Olga Sanchez supports his arguments and says "there is no doubt" Islamic militants were behind the attack.

They are "very near to knowing what happened", she told Cadena SER radio on Wednesday, in her first interview since the attacks.

But representatives of the right-wing Popular Party, which was ousted by the Socialists in elections three days after the bombings, still believe that the Basque separatist group Eta was in some way linked to the attack.

They are refusing to sign up to the interim findings of a parliamentary inquiry, saying it is still unclear who was responsible.

Local and international

More than 70 people have been arrested in the course of the investigations.

The key operational cell is thought by some to have been largely destroyed, with the main players in the attacks either dead, or in jail.

Seven of the bombing suspects died when they blew themselves up during a police raid on the Madrid suburb of Leganes on 3 April, 2004.

The alleged mastermind, a Tunisian named Serhane Ben Abdelmajid Fakhet, died in the Leganes blast.

A suspected explosives expert, Rabei Osman Ahmed, was extradited from Italy in December. Another prime suspect, Jamal Zougam, has been provisionally charged.

A few Spanish-born suspects have been implicated in the supply of explosives, which the bombers packed into 13 rucksacks before leaving them aboard four trains.

In November, one of the Spaniards, a 16-year-old nicknamed "the little gypsy", was sentenced to six years in youth custody for helping to transport the material from northern Spain to Madrid.

Spanish investigators say some of the men arrested in connection with the attacks have ties to terrorist activities outside Spain.

In December, Judge del Olmo charged the alleged leader of a Moroccan group thought to be linked to al-Qaeda with 191 counts of murder. The Moroccan Islamic Combat Group is also blamed for the May 2003 Casablanca bombings.

Wider ring

At least five suspects in the Madrid blasts are thought to remain at large.

Investigators are now looking at suspects who have been charged in other cases with belonging to al-Qaeda or the Moroccan Islamic Combat Group, Reuters news agency reports.

Despite the progress that has been made in dismantling Islamic terrorist cells in Spain and elsewhere in Europe, counterterrorism experts stress that the threat of similar attacks remains.

And experts are said to believe the full extent of the terrorist network involved in the 11 March bombings has yet to be determined.